SILICON ETCH PROCESS WITH TUNABLE SELECTIVITY TO SiO2 AND OTHER MATERIALS

ABSTRACT

A tunable plasma etch process includes generating a plasma in a controlled flow of a source gas including NH 3  and NF 3  to form a stream of plasma products, controlling a flow of un-activated NH 3  that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH 3  until the pressure is within a tolerance of a desired pressure. An etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH 3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of, and claims the benefit of priority to U.S. Provisional Patent Application No. 62/054,860, filed 24 Sep. 2014 and entitled “Silicon Etch Process With Tunable Selectivity to SiO₂ and Other Materials,” which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

The present disclosure applies broadly to the field of plasma processing equipment. More specifically, systems and methods for managing selectivity of plasma etches are disclosed.

BACKGROUND

Semiconductor processing often utilizes plasma processing to etch, clean or deposit material on semiconductor wafers. Plasma etching often targets a particular material, but may also affect other materials that are present on the same wafer, or other workpiece. Selectivity of a plasma etch is often defined as a ratio of an etch rate of a target material to the etch rate of the same etch to another material. It is often advantageous for selectivity to be very high, such that the target material can be etched with little effect on other materials.

SUMMARY

In an embodiment, a tunable plasma etch process includes generating a plasma in a controlled flow of a source gas including NH₃ and NF₃ to form a stream of plasma products, controlling a flow of un-activated NH₃ that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH₃ until the pressure is within a tolerance of a desired pressure. An etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH₃.

In an embodiment, a plasma etch processing system that etches silicon and silicon dioxide with tunable selectivity includes a first NH₃ flow controller, an NF₃ flow controller, and a remote plasma source configured to apply RF energy to a plasma source gas stream controlled by the first NH₃ flow controller and the NF₃ flow controller, to generate a plasma product stream from the plasma source gas stream. The processing system further includes a second NH₃ flow controller configured to control an un-activated NH₃ gas stream, apparatus for adding the un-activated NH₃ gas stream to the plasma product stream to form an etch gas stream, and a process chamber configured to expose a workpiece to the etch gas stream. Etch rates of at least one of polysilicon and silicon dioxide by the etch gas stream on the workpiece are adjustable at least by varying a ratio of the plasma source gas stream to the un-activated NH₃ gas stream.

In an embodiment, a plasma etch processing system includes a process chamber configured to expose a workpiece to an etch gas stream, and a plasma source that generates a plasma product stream from a source gas stream that includes NH₃ and NF₃. The system also includes means for controlling a first NH₃ flow and an NF₃ flow of the source gas stream, and means for controlling a second NH₃ flow of an un-activated NH₃ gas stream. The system also includes a diffuser plate disposed between the plasma source and the process chamber that allows the plasma product stream to flow through the diffuser plate toward the process chamber, and adds the un-activated NH₃ gas stream only on a process chamber side of the diffuser plate, to form an etch gas stream. The system also includes a controller for controlling the plasma source, the means for controlling the first NH₃ flow and the NF₃ flow of the source gas stream, and the means for controlling the second NH₃ flow of the un-activated NH₃ gas stream, such that the controller adjusts etch rates of at least one of polysilicon and silicon dioxide by the etch gas stream on the workpiece by varying a ratio of the un-activated NH₃ gas stream to the source gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates major elements of a plasma processing system, according to an embodiment.

FIG. 2 schematically illustrates major elements of a plasma processing system, in a cross-sectional view, according to an embodiment.

FIG. 3 is a flowchart of a tunable plasma etch process, according to an embodiment.

FIG. 4 shows a graph that illustrates test etch rate results of a Si etch having tunable selectivity to SiO₂, according to an embodiment.

FIG. 5 illustrates an SiO₂ region on a wafer section, forming trenches that are lined with TiN, according to an embodiment.

FIG. 6 illustrates the SiO₂ region of FIG. 5 as including a significant fraction of Si, according to an embodiment.

FIG. 7 illustrates the result of etching the wafer section of FIG. 5 with a Si etch having tunable selectivity to SiO₂ to form a modified wafer section, according to an embodiment.

FIGS. 8A, 8B and 8C schematically illustrate a process sequence in which trace amounts of SiO₂ are not successfully removed by prior art processing.

FIGS. 9A, 9B, 9C and 9D schematically show how the structure shown in FIGS. 8A, 8B and 8C can be more successfully processed with a Si etch with tunable selectivity to oxide, according to embodiments herein.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates major elements of a plasma processing system 100, according to an embodiment. System 100 is depicted as a single wafer, semiconductor wafer plasma processing system, but it will be apparent to one skilled in the art that the techniques and principles herein are applicable to plasma generation systems of any type (e.g., systems that do not necessarily process wafers or semiconductors). It should also be understood that FIG. 1 is a simplified diagram illustrating only selected, major elements of system 100; an actual processing system will accordingly look different and likely contain additional elements as compared with system 100.

Processing system 100 includes a housing 110 for at least a wafer interface 115, a user interface 120, a plasma processing unit 130, a controller 140, one or more flow controller(s) 156 and one or more power supplies 150. Processing system 100 is supported by various utilities that may include gas(es) 155, electrical power 170, vacuum 160 and optionally others; within system 100, controller 140 may control use of any or all of such utilities. Internal plumbing and electrical connections within processing system 100 are not shown, for clarity of illustration.

Processing system 100 is illustrated as a so-called indirect, or remote, plasma processing system that generates a plasma in a first location and directs the plasma and/or plasma products (e.g., ions, molecular fragments, free radicals, energized species and the like) to a second location where processing occurs. Thus, in FIG. 1, plasma processing unit 130 includes a remote plasma source 132 that supplies plasma and/or plasma products for a process chamber 134. Process chamber 134 includes one or more wafer pedestals 135, to which wafer interface 115 delivers and retrieves workpieces 50 (e.g., a semiconductor wafer, but could be a different type of workpiece) for processing. Wafer pedestal 135 is heated and/or cooled by a wafer heating/cooling apparatus 117 which could include, in embodiments, resistive heaters, other types of heaters, or cooling fluids. In operation, gas(es) 155 are introduced into plasma source 132 and a radio frequency generator (RF Gen) 165 supplies power to ignite a plasma within plasma source 132. Plasma and/or plasma products pass from plasma source 132 through at least a diffuser plate 137 to process chamber 134, where workpiece 50 is processed. An actual plasma system may provide many other optional features or subsystems through which plasma and/or plasma products flow and/or mix between plasma source 132 and process chamber 134, and may include sensors 118 that measure parameters such as pressures, temperatures, optical emissions and the like at wafer pedestal 135, within process chamber 134 as shown, and possibly at other locations within processing system 100.

The elements illustrated as part of system 100 are listed by way of example and are not exhaustive. Many other possible elements, such as: pressure and/or flow controllers; gas or plasma manifolds or distribution apparatus; ion suppression plates; electrodes, magnetic cores and/or other electromagnetic apparatus; mechanical, pressure, temperature, chemical, optical and/or electronic sensors; wafer or other workpiece handling mechanisms; viewing and/or other access ports; and the like may also be included, but are not shown for clarity of illustration. In particular, flow controllers may be, or include, mass flow controllers, valves, needle valves, and pressure regulators. Various control schemes affecting conditions in process chamber 105 are possible. For example, a pressure may be maintained by monitoring the pressure in process chamber 134 and adjusting all gas flows upwards or downwards until the measured pressure is within some tolerance of a desired pressure. In embodiments herein, the pressure may be controllable within a range of about 0.5 Torr to 10 Torr; in certain embodiments, controlling pressure around 2 Torr or 6 Torr may be advantageous. Temperatures can be controlled by adding heaters and temperature sensors to process chamber 134 and/or wafer pedestal 135. Optical sensors may detect emission peaks of plasmas as-generated and/or as they interact with workpieces.

Internal connections and cooperation of the elements illustrated within system 100 are also not shown for clarity of illustration. In addition to RF generator 165 and gases 155, other representative utilities such as vacuum 160 and/or general purpose electrical power 170 may connect with system 100. Like the elements illustrated in system 100, the utilities illustrated as connected with system 100 are intended as illustrative rather than exhaustive; other types of utilities such as heating or cooling fluids, pressurized air, network capabilities, waste disposal systems and the like may also be connected with system 100, but are not shown for clarity of illustration. Similarly, while the above description mentions that plasma is ignited within remote plasma source 132, the principles discussed below are equally applicable to so-called “direct” plasma systems that create a plasma in a the actual location of workpiece processing.

Although an indirect plasma processing system is illustrated in FIG. 1 and elsewhere in this disclosure, it should be clear to one skilled in the art that the techniques, apparatus and methods disclosed herein may also be applicable to direct plasma processing systems—e.g., where a plasma is ignited at the location of the workpiece(s). Similarly, in embodiments, the components of processing system 100 may be reorganized, redistributed and/or duplicated, for example: (1) to provide a single processing system with multiple process chambers; (2) to provide multiple remote plasma sources for a single process chamber; (3) to provide multiple workpiece fixtures (e.g., wafer pedestals 135) within a single process chamber; (4) to utilize a single remote plasma source to supply plasma products to multiple process chambers; and/or (5) to provide plasma and gas sources in serial/parallel combinations such that various source gases may be activated (e.g., exist at least temporarily as part of a plasma) zero, one, two or more times, and mixed with other source gases before or after they enter a process chamber, and the like. Gases that have not been part of a plasma are sometimes referred to as “un-activated” gases herein.

FIG. 2 schematically illustrates major elements of a plasma processing system 200, in a cross-sectional view, according to an embodiment. Plasma processing system 200 is an example of plasma processing unit 130, FIG. 1. Plasma processing system 200 includes a process chamber 205 and a plasma source 210. As illustrated in FIG. 2, plasma source 210 introduces gases 155(1) directly, and/or gases 155(2) that become plasma products in a further remote plasma source 202, as plasma input gas 212, through an RF electrode 215. RF electrode 215 includes (e.g., is electrically tied to) a first gas diffuser 220 and a face plate 225 that serve to redirect flow of the source gases so that gas flow is uniform across plasma source 210, as indicated by small arrows 226. After flowing through face plate 225, an insulator 230 electrically insulates RF electrode 215 from a diffuser 235 that is held at electrical ground (e.g., diffuser 235 serves as a second electrode counterfacing face plate 225 of RF electrode 215). Surfaces of RF electrode 215, diffuser 235 and insulator 230 define a plasma generation cavity where a capacitively coupled plasma 245 is created when the source gases are present and RF energy is provided through RF electrode 215. RF electrode 215 and diffuser 235 may be formed of any conductor, and in embodiments are formed of aluminum (or an aluminum alloy, such as the known “6061” alloy type). Surfaces of face plate 225 and diffuser 235 that face the plasma cavity or are otherwise exposed to reactive gases may be coated with yttria (Y₂O₃) or alumina (Al₂O₃) for resistance to the reactive gases and plasma products generated in the plasma cavity. Insulator 230 may be any insulator, and in embodiments is formed of ceramic.

Plasma products generated in plasma 245 pass through diffuser 235 that again helps to promote the uniform distribution of plasma products, and may assist in electron temperature control. Upon passing through diffuser 235, the plasma products pass through a further diffuser 260 that promotes uniformity as indicated by small arrows 227, and enter process chamber 205 where they interact with workpiece 50, such as a semiconductor wafer, atop wafer pedestal 135. Diffuser 260 includes further gas channels 250 that may be used to add one or more additional, un-activated gases 155(3) to the plasma products as they enter process chamber 205, as indicated by very small arrows 229. Diffuser 260 provides un-activated gases 155(3) only on a downstream, or process chamber side, as shown.

Embodiments herein may be rearranged and may form a variety of shapes. For example, RF electrode 215 and diffuser 235 are substantially radially symmetric in the embodiment illustrated in FIG. 2, and insulator 230 is a ring disposed against peripheral areas of face plate 225 and diffuser 235, for an application that processes a circular semiconductor wafer as workpiece 50. However, such features may be of any shape that is consistent with use as a plasma source. Moreover, the exact number and placement of features for introducing and distributing gases and/or plasma products, such as diffusers, face plates and the like, may also vary. For example, it would be possible to substitute an arrangement for generating an inductively coupled plasma, for the capacitively coupled plasma arrangement illustrated. Also, in a similar manner to diffuser 260 including gas channels 250 to add un-activated gas 155(3) to plasma products from plasma 245 as they enter process chamber 205, other components of plasma processing system 200 may be configured to add or mix gases 155 with other gases and/or plasma products as they make their way through the system to process chamber 205.

In semiconductor processing, plasma etch processes for etching polycrystalline silicon (hereinafter sometimes referred to as “polysilicon” or “poly”) are often advantageously selective to silicon over silicon dioxide (hereinafter sometimes referred to as “oxide”). One process scenario in which a plasma etch is highly selective to poly over oxide is as follows. Polysilicon is often utilized as a “gate” electrode in so-called “MOS” transistors (MOS standing for Metal-Oxide-Semiconductor, although poly, instead of metal, is usually used for the gate electrode material). To form planar MOS transistors, a “source” and “drain” are formed in a silicon wafer, with the gate defining separation of, and influencing electrical conduction between, the source and drain. A poly layer may be deposited over a dielectric layer, typically SiO₂. The SiO₂ layer is typically very thin, to maximize field strength in this layer in the final transistor when a voltage is applied. The poly layer is typically thicker than the SiO₂ layer, and poly depositions are often conformal such that minor crevices on the wafer surface fill with poly. When the poly layer is patterned, the poly in these crevices must be removed to prevent adjacent transistor gates from shorting to one another. The SiO₂ layer may need to protect other parts of the wafer during the fabrication process, especially as portions of the poly layer are etched away to form the transistor gates. Therefore, a poly etch with low selectivity to oxide (e.g., a poly etch that etches oxide, as well) might etch through the SiO₂ layer and damage the underlying silicon wafer areas that are intended to form the source and drain of the transistor.

There are, however, device geometries and processes that benefit from a less selective poly etch. For example, recent technologies have moved away from the original, planar MOS process in which gates always overlie thin dielectric layers on a generally flat silicon wafer surface, and/or form capacitors with conductors across a thin dielectric layer. Certain processes etch trenches into silicon surfaces, then grow or deposit oxides on the trench surfaces to form trench devices, including trench capacitors having much larger capacitance per unit area than would be possible on the flat wafer surface. These and/or other processes generate silicon fins by, for example, generating vertical steps on a wafer, depositing poly conformally, and anisotropically etching back the poly in a vertical direction, clearing the poly from horizontal surfaces but leaving the fins behind as residual features against the vertical steps. Many other arrangements are in use or under study to reduce wafer area needed for fabrication of complex devices, to improve device performance, density and/or yield of working circuits per wafer. In such cases, a plasma etch having a selectivity that could be adjustable, or “tunable” from highly selective to poly over oxide, to less selective, or even to being selective to oxide over poly, is advantageous.

Plasma etch embodiments herein have tunable selectivity, for example selectivity to Si over oxide. These embodiments typically utilize NF₃ and NH₃ gases as plasma source gases, and/or as gases that are mixed with plasma products before reaching the workpiece. Certain other gases such as He or Ar may additionally be used as carrier gases but generally do not take part in reactions; also, small percentages (less than about 10% of total gas flow) of oxygen may be added to help with plasma ignition and to boost reaction rates in the plasma. Generally speaking, in the plasma environment NF₃ and NH₃ dissociate, and/or some of the generated products thereof can combine, to form plasma products such as free H and F radicals, HF, N₂, NH₄F and/or NH₄F.HF. Of these plasma products, plasma activation favors formation of free H and F radicals, HF and N₂, while addition of un-activated NH₃ favors formation of NH₄F and/or NH₄F.HF. When Si (e.g., polysilicon) is present, F and Si can react to form SiF₄, while H and Si can react to form SiH₄. When SiO₂ is present, NH₄F or NH₄F.HF can react with SiO₂ to form (NH₄)₂SiF₆ and H₂O; importantly, (NH₄)₂SiF₆ generally forms and remains as a solid, but heat can cause it to dissociate to form SiF₄, NH₃ and HF. Also when SiO₂ is present, NH₄F and NH₄F.HF can react with SiO₂ to form SiH₄F, H₂O and NH₃. Certain of these source gases may participate in reactions even when provided in un-activated form, for example, when provided as un-activated gas flow 155(3), in addition to plasma products from plasma 245, FIG. 2.

Etches described herein are also tunable with respect to selectivity of poly over silicon nitride. (The classic formulation of silicon nitride is Si₃N₄, but stoichiometry of silicon nitride can vary depending on the conditions in which it is generated. In this disclosure, Si₃N₄ and other stoichiometric variations of silicon nitride are referred to herein simply as SiN.) When SiN is present, NF₃ and SiN can react to form NF and SiF₄, while NF₃, NH₃ and SiN can react to form SiH₄.HF and NH₄F.HF.

It has been discovered that in embodiments, certain ones of the input gases (e.g., NF₃ and NH₃) have different effects on the etching, as does the proportion of input gases that are activated by forming plasma therefrom, to un-activated gases (e.g., the proportion of input gas 212 to gas 155(3), see FIG. 2), so as to provide etches with tunable selectivity. Temperature and pressure have also been discovered to have effects. These effects can be utilized to tune an etch that fundamentally uses the same types of input gases to have different selectivities, as discussed below. As noted above, it is possible to adjust ratios of plasma products to un-activated gas (e.g., by adjusting ratios of input gas 212, which forms plasma 245, to gas 155(3), see FIG. 2) to further vary these effects. Furthermore, in embodiments, the etches disclosed can be performed in such a way as to maintain very high selectivity to other materials that may be present on a wafer and should be left undisturbed, such as TiN or W. Still furthermore, the etches disclosed can, in embodiments, have differing etch rates such that an etch recipe can be tuned for high etch rate during one part of an etch sequence, and a different etch rate during another part of the etch sequence.

In an embodiment, a first variation of a plasma etch process (or “recipe”) has an initial ratio of NH₃ to NF₃ as a plasma source gas (e.g., input gas 212) that forms plasma products in plasma 245, and adds further NH₃ as an un-activated gas (e.g., gas 155(3)) resulting in a ratio of input gas 212 to gas 155(3) of about 10:1. The first recipe is performed at a pressure of 2 Torr at the wafer (e.g., workpiece 50 in process chamber 205). The pressure of 2 Torr may be maintained, for example, by monitoring the pressure in process chamber 205 and adjusting all gas flows upwards or downwards until the measured pressure is within some tolerance of 2 Torr. The first recipe has a poly over oxide selectivity of about 20:1. In the first recipe, the dominant plasma reactions include NF₃ and NH₃ providing substantial amounts of free F and H radicals, which react aggressively with Si but less aggressively with SiO₂.

A second variation of the etch recipe flows the same ratio of NH₃ to NF₃ as input gas 212, but at a lower volume, and/or adds more NH₃ as un-activated gas 155(3), as compared to the first recipe. The gas flows of the second etch recipe result in a ratio of input gas 212 to gas 155(3) of about 4:1 or 5:1. The second etch recipe is also performed at a pressure of 2 Torr. The resulting poly over oxide etch rate selectivity is about 2:1. The reduced etch rate selectivity is due to the adjusted gas flows favoring production of NH₄F and NH₄F.HF, which substantially etch SiO₂, over production of F and H radicals, which preferentially etch Si.

A third variation of the etch recipe flows the same ratio of NH₃ to NF₃ as input gas 212, and NH₃ as un-activated gas 155(3), as compared to the second recipe, but is performed at a pressure of 6 Torr. The increased pressure reduces the mean free path of available F and H radicals, which again favors the etching of SiO₂ over the etching of Si. The third etch recipe actually etches oxide faster than poly, with a resulting etch rate selectivity of oxide over poly of about 3:1.

The etch recipes noted above may proceed faster at relatively high wafer temperatures, such as greater than 100 C. The etch recipes noted above can also be highly selective to materials such as TiN and W that are commonly utilized in semiconductor processing at the so-called “frontend” of the process (e.g., layers where transistors and gates are formed and interconnected). That is, the etch rates of TiN and W can be negligibly low. Achieving high selectivity to TiN requires that temperature of a wafer being etched be maintained at less than about 125 C, at which temperature fluorine radicals begin to attack TiN.

Because the etches described above continuously vary the ratio of NH₃ and NF₃ plasma products (e.g., free F and H radicals) to un-activated NH₃ that is added, and the pressure at the workpiece (e.g., in process chamber 205), it is believed that the selectivity of poly to oxide etch rates can be continuously adjusted at least between the extremes noted above; that is, at least from 20:1 poly over oxide to 3:1 oxide over poly. This generates new possibilities for etch recipes, for example, the ability to tailor a plasma etch to provide a selectivity that accommodates process requirements for certain device geometries, or to change the etch selectivity to accommodate changes in device geometry. It may also simplify the deployment of plasma equipment for multiple, similar etch steps in semiconductor process flows, since the input chemistry utilizes the same gases in different ratios to produce different results. Because the types of input gases do not change, process aspects such as conditioning of chamber surfaces is expected not to change significantly, such that a single piece of processing equipment could be shifted rapidly from one process to another with little or no re-conditioning of the surfaces.

FIG. 3 is a flowchart of a tunable plasma etch process 300. Plasma etch process 300 may be implemented, for example, by plasma processing system 100, FIG. 1, and/or other plasma processing systems that utilize plasma processing system 200, FIG. 2. In step 302, a plasma is generated in a controlled flow of a source gas that includes NH₃ and NF₃. An example of step 302 is flowing a mixture of NH₃ and NF₃ as input gas 212, FIG. 2, and generating plasma 245 therefrom. Plasma 245 will include plasma products such as H and F free radicals, HF and N2, as shown in reaction (1) above. In step 306, a controlled flow of un-activated NH₃ is added to the stream of plasma products, to form an etch gas stream. An example of step 306 is flowing NH₃ as un-activated gas 155(3), FIG. 2. Step 310 controls pressure of the etch gas stream by adjusting the source gas and un-activated NH₃ flows until the etch gas pressure is within a tolerance of a desired pressure. An example of step 310 is controlling pressure of the etch gas stream in process chamber 205, FIG. 2, and adjusting flows of input gas 212 and un-activated gas 155(3) until the pressure in process chamber 205 is within a tolerance of the desired pressure. Step 312 adjusts a ratio of source gas to un-activated NH₃ to vary etch rates of polysilicon and/or silicon dioxide to achieve the desired selectivity. Step 312 is done in concert with step 310; that is, an adjustment to one of the source gas and un-activated NH₃ will typically be performed with an adjustment to the other, so that the etch gas pressure and flow ratio are maintained simultaneously.

FIG. 4 shows a graph 330 that illustrates test etch rate results of a Si etch having tunable selectivity to SiO₂. In the etch illustrated, NF₃ and NH₃ may be supplied either as source gases 155(2) in the apparatus illustrated in FIG. 2, and activated in remote plasma source 202, or may be supplied as source gases 155(1) and activated in plasma 245. In the test data illustrated, TiN etch rate was measured but remained at zero across all tested process conditions. When the un-activated NH₃ flow rate was zero, Si had a low etch rate while SiO₂ (labeled as “Oxide” in graph 330) had an etch rate of almost zero. Therefore with an un-activated NH₃ flow rate of zero, the etch conditions were extremely selective to Si over oxide. As the un-activated NH₃ flow rate increased, both Si and oxide etch rates increased, therefore the corresponding etch conditions were less selective to Si over oxide. At the highest un-activated NH₃ flow rate tested, the oxide etch rate increased enough that the Si over oxide selectivity was only about 2:1.

FIGS. 5, 6 and 7 illustrate a process application that advantageously utilizes a Si etch with tunable selectivity to oxide, as described herein.

FIG. 5 illustrates an SiO₂ region 350 on a wafer section 340, forming trenches 380, 385 that are lined with TiN 360. Atop TiN layer 360 is a poly layer 370. A region labeled as A in FIG. 5 is illustrated in greater detail in FIG. 6.

FIG. 6 illustrates that poly layer 370 sits atop a region of SiO₂ 375 that includes a significant fraction of Si. It is important in this process that SiO₂ 375 not be etched significantly, so when SiO₂ 375 is exposed, an etch that is highly selective to poly over oxide should be employed, to minimize attack on SiO₂ 375. Thus, the following discussion begins with generating a plasma in a controlled flow of source gas that includes NH₃ and NF₃, to form a stream of plasma products, and adding a controlled flow of un-activated NH₃ to form an etch gas stream that interacts with poly layer 370 and SiO₂ 375. Referring to FIG. 4, this suggests processing with an un-activated NH₃ flow rate of zero, or nearly zero, to maximize selectivity of the etch to poly over oxide. However, low flows of NH₃ are also associated with somewhat lower Si etch rate, which will make the etch process take longer if the low or zero flow rate of un-activated NH₃ is used continually. A compromise that provides high etch throughput and high selectivity when SiO₂ 375 is exposed, is provided by utilizing the etch with tunable selectivity as described herein. Since the poly etch rate is at least approximately known from graph 330, and the thickness of poly layer 370 is known, the etch can start out with a relatively high un-activated NH₃ flow rate. For example, a flow rate of 150 to 250, in the arbitrary units illustrated in FIG. 4, can be used. This provides a high poly etch rate, and this etch rate is maintained for a time calculated as corresponding to etching partially, but not completely, through the region of poly 370 labeled as 390 in FIG. 6. Then, either in a separate etch step or without interrupting the etch, the un-activated NH₃ flow rate is brought down to zero, or near zero, while etching the region of poly 370 labeled as 395. This causes the poly etch rate to drop, but the corresponding oxide etch rate also drops to near zero, as illustrated in FIG. 4, so the etch can completely clear poly 370 without significant attack on Si rich SiO₂ 375. All of the process conditions are also highly selective to TiN such that a reasonable overetch can be used to clear poly in topologically difficult locations such as sidewalls of trenches 380 and 385 while leaving TiN 360 intact. FIG. 7 illustrates the result of etching wafer section 340 to form wafer section 340′.

FIGS. 8A, 8B and 8C schematically illustrate a process sequence in which trace amounts of SiO₂ are not successfully removed by prior art processing. FIG. 8A shows a wafer section 400 that includes a portion of a silicon substrate 410 and overlying structures. A spacer structure 450 is part of a fin-FET process. Certain recesses within spacer structure 450 contain inter-layer dielectric oxides 430 that are generally to be retained in the etch sequence to be described. Other recesses contain polysilicon 420 that is to be removed; however the polysilicon is partially oxidized along grain boundaries thereof, shown as oxide traces 425, near the surface. Depth of oxide traces 425 is known with reasonable accuracy. Below the poly, adjacent to silicon substrate 410 in the same recesses of spacer structure 450 are thin layers of high dielectric constant oxide (“high-K oxide” hereinafter) 440 that are not to be etched at all.

FIG. 8B schematically illustrates partial etching of wafer section 400 with a highly selective poly etch to form a partially processed wafer section 401. The poly etch used is highly selective so as not to etch high-K oxide 440, but because of this selectivity, oxide traces 425 are also not etched. Wafer section 401 shows how wafer section 401 will look as poly 420 is about one third etched away.

FIG. 8C schematically illustrates further etching of wafer section 400 with a highly selective poly etch to form a partially processed wafer section 402. Some oxide traces 425 remain in their original locations, while others, having lost all mechanical connection with spacer structure 450, migrate about the recesses to other locations, as shown. At least some of oxide traces 425 cause electrical defects in the finished semiconductor product, as they interfere with the structures that should be present at their locations, and/or cause photolithographic defects in further processing.

FIGS. 9A, 9B, 9C and 9D schematically show how the structure shown as wafer section 400 can be more successfully processed with a Si etch with tunable selectivity to oxide, according to embodiments herein. Thus, the following discussion begins with generating a plasma in a controlled flow of source gas that includes NH₃ and NF₃, to form a stream of plasma products, and adding a controlled flow of un-activated NH₃ to form an etch gas stream that interacts with the features of wafer section 400. FIG. 9A shows wafer section 400 (identical to that shown in FIG. 8A). FIG. 9B illustrates partial etching of wafer section 400 with the etch gas stream, to form a partially processed wafer section 403. The etch gas stream used to form wafer section 403 includes enough NH₃ to provide a nonzero oxide etch rate, that is, to make the etch only partially selective to poly over oxide. As shown in FIG. 9B, oxide traces 425 are etched along with poly 420 in this part of the etch process. Once the etch penetrates past oxide traces 425 (for example, after a time at which a measured or calculated etch rate ensures etching to or beyond the known maximum depth of oxide traces 425) NH₃ flow can be reduced to zero or near zero, to increase selectivity of poly over oxide. FIG. 9C illustrates partial etching of wafer section 400 with the etch, to form a partially processed wafer section 403. The etch proceeds in this manner until poly 420 is completely removed, as shown in wafer section 405, FIG. 9D. The high selectivity of the Si etch having tunable selectivity to oxide ensures that high-K oxide 440 is not damaged as poly 420 is removed.

Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth. Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups. 

1. A tunable plasma etch process, comprising: generating a plasma in a controlled flow of a source gas including NH₃ and NF₃ to form a stream of plasma products; controlling a flow of un-activated NH₃ that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH₃ until the pressure is within a tolerance of a desired pressure; wherein an etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH₃.
 2. The tunable plasma etch process of claim 1, wherein the etch rates of polysilicon and silicon dioxide by the etch gas stream are adjustable at least across a range of: 20:1 selectivity of the polysilicon etch rate over the silicon dioxide etch rate, to 2:1 selectivity of the polysilicon etch rate over the silicon dioxide etch rate.
 3. The tunable plasma etch process of claim 1, wherein adjusting the ratio of the controlled flow of the source gas to the flow of un-activated NH₃, from 10:1 to 5:1 reduces the selectivity of the polysilicon etch rate over the silicon dioxide etch rate from 20:1 to 2:1.
 4. The tunable plasma etch process of claim 1, wherein decreasing the ratio of the controlled flow of the source gas to the flow of un-activated NH₃ reduces a concentration of at least one of F and H radicals in the etch gas stream.
 5. The tunable plasma etch process of claim 1, wherein a ratio of NF₃ to NH₃ in the source gas is greater than one.
 6. The tunable plasma etch process of claim 1, wherein generating the plasma comprises generating a capacitively coupled plasma.
 7. The tunable plasma etch process of claim 1, wherein the pressure is controllable at least across a range of 0.5 Torr to 10 Torr.
 8. The tunable plasma etch process of claim 7, wherein the ratio of the controlled flow of the source gas to the flow of un-activated NH₃ is 5:1, the pressure is controlled to 6 Torr and the polysilicon and silicon dioxide etch rates are such that an etch selectivity of the tunable plasma etch process is about 3:1, silicon dioxide over poly.
 9. The tunable plasma etch process of claim 1, further comprising adding oxygen to the source gas, prior to generating the plasma.
 10. The tunable plasma etch process of claim 1, further comprising adding one of helium and argon to the source gas, prior to generating the plasma.
 11. The tunable plasma etch process of claim 1, further comprising controlling a temperature of a workpiece on which the etch rates of the at least one of polysilicon and silicon dioxide are adjusted.
 12. The tunable plasma etch process of claim 11, further comprising controlling the temperature of the workpiece within a range of 100 C to 125 C. 13-20. (canceled) 